As is known in the art, air traffic control is a service to promote the safe, orderly, and expeditious flow of air traffic. Safety is principally a matter of preventing collisions with other aircraft, obstructions, and the ground; assisting aircraft in avoiding hazardous weather; assuring that aircraft do not operate in airspace where operations are prohibited; and assisting aircraft in distress. Orderly and expeditious flow assures the efficiency of aircraft operations along the routes selected by the operator. It is provided through the equitable allocation of system resources to individual flights, generally on a first-come-first-served basis.
As is also known, air traffic control services are provided by air traffic control systems. Air traffic control systems are a type of computer and display system that processes data received from air surveillance radar systems for the detection and tracking of aircraft. Air traffic control systems are used for both civilian and military applications to determine the identity and locations of aircraft in a particular geographic area. Such detection and tracking is necessary to notify aircraft flying in proximity of each other and to warn aircraft which appear to be on a collision course. This is particularly true around airports where a relatively large number of aircraft fly in a relatively small geographic area. It is also desirable to track aircraft to determine and verify that particular aircraft are travelling along particular routes. In this manner it is possible to reduce collisions between aircraft. Air traffic control systems thus generally provide information on aircraft in the vicinity of airports from data provided by air-surveillance radars, as well as information on aircraft travelling between airports from data provided by air-route surveillance radars.
Air-traffic controller systems use one or more radar systems to monitor the positions of aircraft in their sectors of responsibility and to monitor areas of heavy precipitation. Each of the radar systems typically include an antenna, a transmitter and a receiver. The radar information is used to develop clearances and instructions for separating aircraft operating on flight plans under instrument flight rules, and to provide traffic advisories to instrument-flight-rules aircraft and to visual-flight-rules aircraft receiving the traffic advisory service. Two principal types of radar are used in civil air-traffic control, secondary, or beacon, radar and primary radar.
A secondary radar refers to an interrogate-respond system. In this type of system, a directional antenna located at a ground station transmits a pulse pair to a transponder in the aircraft. The pulse spacing encodes one of two messages, "transmit your altitude" (the Mode C interrogation) or "transmit your identity" (the Mode 3/A interrogation). The aircraft transponder transmits an encoded pressure-altitude reply in response to the first interrogation and a four-digit identity code, assigned by air-traffic control and entered into the transponder by the pilot, in response to the second. The aircraft is shown on the controller's plan view display at the azimuth corresponding to the pointing direction of the antenna and the range corresponding to the round-trip time between transmission of the interrogation and receipt of the reply. Air-traffic control computers receive the encoded reply data from radar sites and place corresponding information in data blocks next to the symbols depicting the aircraft positions on the display. The identity code assigned by air-traffic control is correlated with the database in the computer of flight plans filed under instrument flight rules to display the radio call sign in the data block. The aircraft pressure altitude is displayed in hundreds of feet.
A primary radar, on the other hand, operates by transmitting a relatively high-power, radio-frequency (RF) pulse from a directional antenna. The energy is reflected from any aircraft in the directional beam and received by the antenna. The aircraft is displayed at the azimuth corresponding to the pointing direction of the antenna and the range corresponding to the round-trip time between pulse transmission and receipt of the reflected signal.
The radar systems are typically coupled to an air traffic control automation (ATCA) system.
Each of the one or more radar systems feeds the target data signals to the ATCA system. The ATCA system often includes multiple processors each of which process the target data signals in a particular manner. Among other things, the ATCA system maintains and updates the target data fed thereto to thus maintain accurately the location and speed of targets detected and tracked by the radar system portion of the air traffic control system. In performing this function, the ATCA system typically assigns a unique identifier or "label" to each target that is being tracked.
In conventional air traffic control systems, new radar reports are correlated with aircraft tracks on the basis of discriminants, such as the above-mentioned discrete beacon codes or Mode C responses. If none of these discriminants exist, correlation of the new radar reports to the aircraft tracks is performed by a nearest neighbor computational technique wherein a target is correlated on the basis of its proximity to its track predicted position. When two targets correlate with two tracks, the distance between each target's reported position and each track's predicted position is calculated.
A correlation is established between the target and track which are separated by the shortest distance, with the remaining target and track forming the other correlation. Unfortunately, this correlation method may produce ambiguous results when a target's distance from one track is the same, within measurement accuracy, as its distance from the other track. Such a situation can occur shortly before and after two targets reach a crossing point, in what is named the crossing region.
In the crossing region, there are times when even discrete beacon codes or Mode C targets may not correlate unambiguously, because beacon codes and Mode C responses may be garbled. In this case, the correlation technique defaults to the nearest neighbor computational technique. Nevertheless, a large crossing region, in which multiple miscorrelations may occur, can lead to track equalization, permanent label swapping, and, ultimately, track loss.
One problem which thus arises when multiple targets are tracked by the air traffic control system is the ability to maintain and update target data on multiple targets which fly in proximity to each other thereby preventing the reliable identification of the individual targets. In this case, the labels of crossing targets can frequently be swapped if the targets have no distinguishable attributes. Such targets are Primary Surveillance Radar (PSR) reports and identical Secondary Surveillance radar (SSR) beacon codes with no reported altitude (Mode C). Label swapping is caused by inherent weaknesses in the nearest neighbor techniques used to correlate new radar reports with established tracks.
When a target and track are correlated on the basis of the distance between them, successful correlation is predicated on the condition that this distance does not exceed a threshold value, known as the radius of the correlation search area. In other words, a target correlates with a track only if it is located inside a search area (gate) of a predetermined radius, centered about the track's predicted position. When two tracks cross, their search areas overlap. If, while they overlap, at least one target is found to be in both areas, a potential for label swap exists. When a nearest neighbor technique is being used, the targets are said to be in an ambiguous correlation region (ACR).
To minimize the ACR, the size of the correlation search area should be minimized. However, the smaller the search area, the higher the potential for track loss when the targets emerge from the ACR, because as a result of miscorrelation-caused track degradation, the distance between each target and its track increases, and the target may no longer fall inside the search area. In practice, if search area size is the only means of controlling the correlation process, prevention of label swaps and track losses are mutually exclusive objectives.
It would, therefore, be desirable to provide a technique which allows an increase of the search area size to prevent track loss without increasing the swap probability. It would also be desirable to provide a system which correlates new radar data with existing aircraft tracks and which minimizes miscorrelations, track equalization, permanent label swapping and target track loss.